SEA-FISHERIES LABORATORY. 321 



external and inferior recti enter the orbit, and where the 

 cavity of the latter is prolonged backwards for a littie 

 distance round the recti. 



The recessus of the upper or left orbit has similar 

 relations, but on account of the almost complete enclosure 

 of the latter by bony structures it has a somewhat different 

 position. It lies on the eyeless side of the head external 

 to the fenestra bounded by the left frontal, left prefrontal, 

 parasphenoid and alisphenoid. It must therefore be dis- 

 sected from that side and is easily exposed by simply 

 removing the skin and a little surrounding connective 

 tissue. It is (in a preserved fish of 22 inches long) a 

 round flattened sac of about 2cm. in diameter, of similar 

 appearance and structure to the organ of the right side. 

 It also opens into the orbit, and on removing the left eye 

 the large aperture is easily seen immediately posterior and 

 slightly above the place of origin of the inferior oblique 

 muscle. It is situated beneath the pseudomesial ridge 

 and pierces the soft wall of the fenestra mentioned. 



We have stated that the eyeball can be protruded 

 from the general surface of the head to a remarkable 

 extent. Now while the eye muscles provide an effective 

 apparatus for the retraction of the eye, there is apparently 

 no muscular arrangement which can bring about protru- 

 sion. This appears to be the function of the recessus. In 

 life the cavities of the orbit and that of the recessus with 

 which the latter is in free communication are filled by 

 fluid which apparently originates by an infiltration of 

 lymph from the capillaries outside the orbital wall. . The 

 wall of the recessus being markedly muscular, it follows 

 that its contraction will expel the contained fluid into the 

 orbit and press on the internal surface of the eye-ball. 

 Since the skin of the head round the eye is loose it yields, 

 and the eye is accordingly protruded. Conversely the 

 7< 



